Seated workers in the industrial and office environment often experience back pain and other physiological difficulties as a result of prolonged sitting in a fixed posture. Much research and analysis has been conducted into so-called ergonomic chair design to provide solutions to these difficulties.
Four principal chair designs have been identified and theories have been developed to support each design as solving some seating difficulty. Chairs embodying these designs are available in the market place. One such design provides a flat, horizontal seat pan and a vertically-upright back, enabling a worker to sit with his or her thighs and back at a 90 degree angle to one another. A second chair design, often called a "Mandal" chair provides a seat pan having a forward, downward tilt with an upright back. A third chair design, often called a "Grandjean" chair, provides a back having a backward tilt with a horizontal seat pan. A fourth chair design, often called a "Balans" chair, provides a seat pan having a forward tilt with a knee rest having a backward tilt. A fifth chair design, often called a "standing" chair, provides an elevated seat pan having a forward tilt for a worker that would be partially sitting and standing at the same time; as one might do at a drafting table. All of these designs can be provided with adjustable seat pans and, where used, adjustable back rests having heights to accommodate workers of different body builds and to accommodate different working environments. There appear to be clear advantages and disadvantages to all five postural options afforded by these five chair designs.
All five designs are intended to restore lumbar lordosis to the seated worker, and thereby reduce the pressure on lumbar disks that is a root cause of lower back pain. The various theories simply represent a disagreement as to how restoration of lumbar lordosis might best be achieved. In actual testing of these chair designs, however, researchers have discovered the workers exhibit so-called idiosyncrasies in seated posture; that is to say, workers exhibit working seated postures that do not conform to any of the theories, and that these idiosyncrasies will be influenced by individual variations in both weight and physical fitness of a worker's body parts. In other words, workers do not always sit the way the researchers think and suggest that they ought. It has been concluded that actual sitting behavior is subject to individual variability because of variations in body build, personal habit and personal preference, and training. Seating postures can also be affected by the working environment; whether a worker is engaged at a drafting table, microscope, computer terminal, and so forth. Furthermore, it has been concluded that various physiological recommendations as to appropriate seating postures can be at odds with one another. For example, seating postures that are best for maintaining spinal lordosis may not be the most comfortable or healthy if blood supply to the lower legs is restricted. Workers, for example, when given an opportunity to pick their most comfortable sitting posture, do not always choose a sitting posture that achieves lordosis.
It has been suggested, therefore, that a suitable chair, in order to fit into its working environment, should be adjustable in the following ways: (1) the seat height should be adjustable; (2) seat depth should be adequate for large people without being too large for smaller workers; (3) seat padding should be adequate to allow pressure to be distributed but not so soft that the seat bones of the pelvis (the ischial tuberosities) bottom out; (4) the seat pan should be capable of tilting forward and backward; (5) the back should be alternately positionable both upright and with a backward tilt; (6) the backrest should be padded and have a lumbar support; (7) the chair should have both a dynamic range of posture movements without the worker making adjustments, and multiple static posture settings which can be engaged to lock the chair into any position set by the worker.